Organic light-emitting diode with enhanced efficiency

ABSTRACT

Generally, the devices provided herein comprise at least a hole-transport layer, two light-emitting layers, and an electron-transport layer, each having a highest occupied molecular orbital (HOMO) energy level and a lowest unoccupied molecular orbital (LUMO) energy level, wherein at least one of the HOMO energy levels and/or the LUMO energy levels of at least one of the light-emitting layers does not decrease in a stepwise fashion.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/301,694, filed Feb. 5, 2010, which is incorporated by referenceherein in its entirety.

BACKGROUND

1. Field

The embodiments described herein relate to organic light-emittingdiodes, such as organic light-emitting diodes comprising ahole-transport layer, two light-emitting layers, and anelectron-transport layer.

2. Description of the Related Art

White organic light emitting devices (WOLED) are becoming increasinglyimportant for lighting applications. For example, WOLEDs may be able toreplace fluorescent tubes to save energy. Thus there is a continuingneed to improving the power efficiency of WOLEDs.

Many of the current WOLEDs comprise a hole-transport layer, at least twolight-emitting layers, and an electron-transport layer arranged in thatorder. In these devices, each layer has a highest occupied molecularorbital (HOMO) energy level and a lowest unoccupied molecular orbital(LUMO) energy level, wherein the HOMO energy levels and/or the LUMOenergy levels decrease in a stepwise fashion. In other words, with theenergy levels of the first light-emitting layer are lower than thecorresponding energy levels of the hole-transport layer (e.g. the HOMOof first light-emitting layer is lower than the HOMO of thehole-transport layer, the LUMO of the first light-emitting layer islower than the LUMO of the hole-transport layer), the energy levels ofthe second light-emitting layer are lower than the corresponding energylevels of the first light-emitting layer, and the energy levels of theelectron-transport layer are lower than the corresponding energy levelsof the second light-emitting layer. These devices may suffer from theproblems of electron leakage from the light-emitting layers through thehole-transport layer to the anode, and hole leakage from thelight-emitting layers through the electron-transport layer to thecathode, thus reducing the device efficiency. Traditionally,hole-blocking and electron-blocking layers have sometimes been used toattempt to address this problem, but the additional layers add expenseand complexity to the device fabrication and may reduce deviceefficiency.

Other devices may utilize hole-transport layers with very high LUMOs toblock electron leakage to the anode an/or electron-transport layers withvery low HOMOs to block hole leakage to the cathode. Unfortunately, thelarge energy gap between the corresponding molecular orbital of thehole-transport layer or the electron-transport layer and thecorresponding electrode can significantly reduce hole or electronmobility. The reduced mobility can in turn cause reduced efficiency ofthe device. The large energy gap may also cause higher driving voltage.Thus, it is difficult to improve efficiency using this approach.

Thus, additional options for addressing these problems are needed.

SUMMARY

Generally, the devices provided herein comprise at least ahole-transport layer, two light-emitting layers, and anelectron-transport layer, arranged in that order. In these devices, eachlayer has a highest occupied molecular orbital (HOMO) energy level and alowest unoccupied molecular orbital (LUMO) energy level, and at leastone of the HOMO energy levels and/or the LUMO energy levels of at leastone of the light-emitting layers does not decrease in a stepwisefashion. In other words, one of the energy levels of the firstlight-emitting layer is not lower than one of the corresponding energylevels of the hole-transport layer (e.g. the HOMO of firstlight-emitting layer is not lower than the HOMO of the hole-transportlayer and/or the LUMO of the first light-emitting layer is not lowerthan the LUMO of the hole-transport layer), one of the energy levels ofthe second light-emitting layer is not lower than one of thecorresponding energy levels of the first light-emitting layer, and/orone of the energy levels of the electron-transport layer is not lowerthan one of the corresponding energy levels of the second light-emittinglayer.

For example, some embodiments provide an organic light-emitting devicecomprising: a cathode, an anode, and a series of organic layers disposedbetween the anode and the cathode, wherein the series of organic layerscomprises: a first light-emitting layer deposited between the anode andthe cathode, wherein the first light-emitting layer comprises a firsthost material and a first emissive material; a hole-transport layer,disposed between the anode and the first light-emitting layer a secondlight-emitting layer disposed between the first light-emitting layer andthe cathode, wherein the second light-emitting layer comprises a secondhost material and a second emissive material; and an electron-transportlayer disposed between the second light-emitting layer and the cathode;wherein a HOMO energy level of the hole-transport layer is higher than aHOMO energy level of the electron-transport layer, and a LUMO energylevel of the hole-transport layer is higher than a LUMO energy level ofthe electron transport layer; and at least one of the followingrelationships exists: a HOMO energy level of the first light-emittinglayer is higher than the HOMO energy level of the hole-transport layer;a LUMO energy level of the first light-emitting layer is lower than aLUMO energy level of the second light-emitting layer; a HOMO energylevel of the second light-emitting layer is higher than the HOMO energylevel of the first light-emitting layer; or the LUMO energy level of thesecond light-emitting layer is lower than a LUMO energy level of theelectron-transport layer.

These and other embodiments are described in greater detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 provides a schematic of the structure of some embodiments of adevice described herein.

FIG. 2 depicts the highest occupied molecular orbital (HOMO) energylevels and lowest unoccupied molecular orbital (LUMO) energy levels forthe different layers, the anode work function, and the cathode workfunction for some embodiments.

FIG. 3 depicts the HOMO energy levels and LUMO energy levels for thedifferent layers, the anode work function, and the cathode work functionfor some embodiments.

FIG. 4 depicts the HOMO energy levels and LUMO energy levels for thedifferent layers, the anode work function, and the cathode work functionfor some embodiments.

FIG. 5 depicts the HOMO energy levels and LUMO energy levels for thedifferent layers, the anode work function, and the cathode work functionfor some embodiments.

FIG. 6 depicts the HOMO energy levels and LUMO energy levels for thedifferent layers, the anode work function, and the cathode work functionfor some embodiments.

FIG. 7 depicts the HOMO energy levels and LUMO energy levels for thedifferent layers, the anode work function, and the cathode work functionfor some embodiments.

FIG. 8 depicts the HOMO energy levels and LUMO energy levels for thedifferent layers, the anode work function, and the cathode work functionfor some embodiments.

FIG. 9 is a plot of the luminance efficiency (cd/A) and the powerefficiency (lm/w) as function of brightness (cd/m2) of an embodiment ofa device described herein.

FIG. 10 is a plot of the luminance efficiency (cd/A) and the powerefficiency (lm/w) as function of brightness (cd/m2) of an embodiment ofa device described herein.

FIG. 11 a is a plot of the luminance efficiency (cd/A) and the powerefficiency (lm/w) as function of brightness (cd/m2) of an embodiment ofa device described herein.

FIG. 11 b is a plot of the electroluminescence spectrum of an embodimentof a device described herein.

FIG. 12 a is a plot of the luminance efficiency (cd/A) and the powerefficiency (lm/w) as function of brightness (cd/m2) of an embodiment ofa device described herein.

FIG. 12 b is a plot of the electroluminescence spectrum of an embodimentof a device described herein.

DETAILED DESCRIPTION OF SOME EMBODIMENTS

FIG. 1 provides a schematic of the structure of some embodiments of adevice described herein. These devices comprise an anode 10 and acathode 60, and a series of organic layers 5 disposed between the anode10 and the cathode 60. In these embodiments, the organic layers 5comprise at least a hole-transport layer 20, a first light-emittinglayer 30, a second light-emitting layer 40, and electron-transport layer50, positioned in the order depicted. In some embodiments, the devicemay be configured to allow holes to be transported from the anode to thefirst light-emitting layer and the second light-emitting layer and allowelectrons to be transported from the cathode to the first light-emittinglayer and the second light-emitting layer. The organic layer mayoptionally further comprise one or more of the following: a firstoptional layer 15, disposed between the anode 10 and the hole-transportlayer; a second optional layer 25, disposed between the hole-transportlayer 20 and the first light-emitting layer 30; a third optional layer35, disposed between the first light-emitting layer 30 and the secondlight-emitting layer 40; a fourth optional layer 45, disposed betweenthe second light-emitting layer 40 and the electron-transport layer 50;and a fifth optional layer 55, disposed between the electron-transportlayer 50 and the cathode 60. These optional layers may be any type oflayer such as a hole-injecting layer, a hole-blocking layer, anexciton-blocking layer, an electron-injecting layer, anelectron-blocking layer, etc. In some embodiments, the first optionallayer 15, if present, the second optional layer 25, if present, and thethird optional layer 35, if present, are independently selected from ahole-injecting layer, an electron-blocking layer, and anexciton-blocking layer. In some embodiments, the third optional layer35, the fourth optional layer 45, and the fifth optional layer 55, ifpresent, are independently selected from an electron-injecting layer, ahole-blocking layer, and an exciton-blocking layer. In some embodiments,the first optional layer 15 is a hole-injecting layer. In someembodiments, the fifth optional layer 55 is an electron-injecting layer.

FIGS. 2-8 depict the highest occupied molecular orbital (HOMO) energylevels and lowest unoccupied molecular orbital (LUMO) energy levels forthe different layers, the anode work function 10V, and the cathode workfunction 60V for some embodiments. The terms “highest occupied molecularorbital energy level” or “HOMO energy level” have the ordinary meaningunderstood by a person of ordinary skill in the art. In someembodiments, the HOMO energy level of a material includes the energylevel of the highest energy molecular orbital of that material that isoccupied with at least one electron in the ground state. The terms“lowest unoccupied molecular orbital energy level” or “LUMO energylevel” have the ordinary meaning understood by a person of ordinaryskill in the art. In some embodiments, the LUMO energy level of amaterial includes the energy level of the lowest energy molecularorbital of that material that contains no electrons in the ground state.The “work function” of a metal or electrical conductor is a measure ofthe minimum energy required to extract an electron from the surface ofthe metal or conductor.

In these embodiments, the HOMO energy level 20H of the hole-transportlayer 20 (HT) is higher than the HOMO energy level 50H of theelectron-transport layer 50 (ET), and the LUMO energy level 20L of thehole-transport layer 20 is higher than the LUMO energy level 50L of theelectron transport layer 50. In some embodiments, the anode 10 workfunction 10V may be higher than the HOMO energy level 20H of thehole-transport layer 20, and the work function of the 60V cathode 60 maybe lower than the LUMO energy level 50L of the electron transport layer50.

With respect to FIG. 2, in some embodiments, the HOMO energy level 30Hof the first light-emitting layer 30 (EM-1) may be higher than the HOMOenergy level 20H of the hole-transport layer 20. For example, the HOMOenergy level 30H of the first light-emitting layer 30 (EM-1) may behigher than the HOMO energy level 20H of the hole-transport layer 20 byabout 0.05 eV to about 0.70 eV, about 0.1 eV to about 0.3 eV, or about0.1 eV to about 0.2 eV. In some embodiments, 1, 2, 3, 4, or 5 of thefollowing relationships exist: 1) the HOMO energy level 50H of theelectron-transport layer 50 is lower than the HOMO energy level 40H ofthe second light-emitting layer 40 (EM-2), 2) the HOMO energy level 40Hof the second light-emitting layer 40 is lower than the HOMO energylevel 20H of the hole-transport layer 20, 3) the LUMO energy level 50Lof the electron-transport layer 50 is lower than the LUMO energy level40L of the second light-emitting layer 40, 4) the LUMO energy level 40Lof the second light-emitting layer 40 is lower than the LUMO energylevel 30L of the first light-emitting layer 30, and 5) the LUMO energylevel 30L of the first light-emitting layer 30 is lower than the LUMOenergy level 20L of the hole-transport layer 20.

With respect to FIG. 3, in some embodiments, the LUMO energy level 30Lof the first light-emitting layer 30 may be lower than the LUMO energylevel 40L of the second light-emitting layer 40. For example, the LUMOenergy level 30L of the first light-emitting layer 30 may be lower thanthe LUMO energy level 40L of the second light-emitting layer 40 by about0.05 eV to about 0.70 eV, about 0.1 eV to about 0.3 eV, or about 0.1 eVto about 0.2 eV.. In some embodiments, 1, 2, 3, 4, or 5 of the followingrelationships exist: 1) the HOMO energy level 50H of theelectron-transport layer 50 is lower than the HOMO energy level 40H ofthe second light-emitting layer 40, 2) the HOMO energy level 40H of thesecond light-emitting layer 40 is lower than the HOMO energy level 30Hof the first light-emitting layer 30, 3) the HOMO energy level 30H ofthe first light-emitting layer 30 is lower than HOMO energy level 20H ofthe hole-transport layer 20, 4) the LUMO energy level 50L of theelectron-transport layer 50 is lower than the LUMO energy level 40L ofthe second light-emitting layer 40, and 5) the LUMO energy level 40L ofthe second light-emitting layer 40 is lower than the LUMO energy level20L of the hole-transport layer 20.

With respect to FIG. 4, in some embodiments the HOMO energy level 40H ofthe second light-emitting layer 40 may be higher than the HOMO energylevel 30H of the first light-emitting layer 30. For example, the HOMOenergy level 40H of the second light-emitting layer 40 may be higherthan the HOMO energy level 30H of the first light-emitting layer 30 byabout 0.05 eV to about 0.70 eV, about 0.1 eV to about 0.3 eV, or about0.1 eV to about 0.2 eV. In some embodiments, 1, 2, 3, 4, or 5 of thefollowing relationships exist: 1) the HOMO energy level 50H of theelectron-transport layer 50 is lower than the HOMO energy level 30H ofthe first light-emitting layer 30, 2) the HOMO energy level 30H of thefirst light-emitting layer 30 is lower than the HOMO energy level 20H ofthe hole-transport layer 20, 3) the LUMO energy level 50L of theelectron-transport layer 50 is lower than the LUMO energy level 40L ofthe second light-emitting layer 40, 4) the LUMO energy level 40L of thesecond light-emitting layer 40 is lower than the LUMO energy level 30Lof the first light-emitting layer 30, and 5) the LUMO energy level 30Lof the first light-emitting layer 30 is lower than the LUMO energy level20L of the hole-transport layer 20.

With respect to FIG. 5, in some embodiments the LUMO energy level 40L ofthe second light-emitting layer 40 may be lower than the LUMO energylevel 50L of the electron-transport layer 50. For example, the LUMOenergy level 40L of the second light-emitting layer 40 may be lower thanthe LUMO energy level 50L of the electron-transport layer 50 by about0.05 eV to about 0.70 eV, about 0.1 eV to about 0.3 eV, or about 0.1 eVto about 0.2 eV. In some embodiments, 1, 2, 3, 4, or 5 of the followingrelationships exist: 1) the HOMO energy level 50H of theelectron-transport layer 50 is lower than the HOMO energy level 40H ofthe second light-emitting layer 40, 2) the HOMO energy level 40H of thesecond light-emitting layer 40 is lower than the HOMO energy level 30Hof the first light-emitting layer 30, 3) the HOMO energy level 30H ofthe first light-emitting layer 30 is lower than HOMO energy level 20H ofthe hole-transport layer 20, 4) the LUMO energy level 50L of theelectron-transport layer 50 is lower than the LUMO energy level 30L ofthe first light-emitting layer 30, and 5) the LUMO energy level 30L ofthe first light-emitting layer 30 is lower than the LUMO energy level20L of the hole-transport layer 20.

The combination of relationships between the various HOMO and LUMOenergy levels with respect to FIGS. 2-5 provides a variety of differentenergetic structures. For example, in FIG. 6, the HOMO energy level 40Hof the second light-emitting layer is higher than the HOMO energy level30H of the first light-emitting layer 30, and the LUMO energy level 30Lof the first light-emitting layer 30 is lower than the LUMO energy level40L of the second light-emitting layer 40. Furthermore, in someembodiments, the HOMO energy level 50H of the electron-transport layer50 is lower than the HOMO energy level 30H of the first light-emittinglayer 30, the HOMO energy level 30H of the first light-emitting layer 30is lower than the HOMO energy level 20H of the hole-transport layer 20,the LUMO energy level 50L of the electron-transport layer 50 is lowerthan the LUMO energy level 40L of the second light-emitting layer 40,and the LUMO energy level 40L of the second light-emitting layer 40 islower than the LUMO energy level 20L of the hole-transport layer 20.

FIG. 7 depicts the energetic structure of some embodiments. In thesedevices, the HOMO energy level 40H of the second light-emitting layer 40is higher than the HOMO energy level 30H of the first light-emittinglayer 30, and the LUMO energy level 40L of the second light-emittinglayer 40 is lower than the LUMO energy level 50L of theelectron-transport layer 50. Furthermore, in some embodiments, the HOMOenergy level 50H of the electron-transport layer 50 is lower than theHOMO energy level 30H of the first light-emitting layer 30, the HOMOenergy level 30H of the first light-emitting layer 30 is lower than theHOMO energy level 20H of the hole-transport layer 20, the LUMO energylevel 50L of the electron-transport layer 50 is lower than the LUMOenergy level 30L of the first light-emitting layer 30, and the LUMOenergy level 30L of the first light-emitting layer 30 is lower than theLUMO energy level 20L of the hole-transport layer 20.

FIG. 8 depicts the energetic structure of some embodiments. In thesedevices, the HOMO energy level 40H of the second light-emitting layer 40is higher than the HOMO energy level 30H of the first light-emittinglayer 30, the LUMO energy level 30L of the first light-emitting layer 30is lower than the LUMO energy level 40L of the second light-emittinglayer 40, and the LUMO energy level 40L of the second light-emittinglayer 40 is lower than the LUMO energy level 50L of theelectron-transport layer 50.

The cathode may include any material having a lower work function thanthe anode layer. Examples of suitable materials for the cathode layerinclude those selected from alkali metals of Group 1; Group 2 metals;Group 3 metals; Group 12 metals including rare earth elements,lanthanides and actinides; materials such as aluminum, indium, calcium,barium, samarium and magnesium; and combinations thereof. Li-containingorganometallic compounds, LiF, and Li₂O may also be deposited betweenthe organic layer and the cathode layer to lower the operating voltage.Suitable low work function metals include but are not limited to Al, Ag,Mg, Ca, Cu, Mg/Ag, LiF/Al, CsF, CsF/Al or alloys thereof. In someembodiments, the cathode layer can have a thickness in the range ofabout 1 nm to about 1000 nm.

Approximate work functions of some materials which may be useful in acathode are included in Table 1 below.

TABLE 1 Metal Work Function (eV) LiF/Al 2.6 Mg 3.72 Mg/Ag 4.12 Al 4.28

The anode layer may comprise any material having a higher work functionthan the cathode layer. In some embodiments, the anode layer maycomprise a conventional material such as a metal, mixed metal, alloy,metal oxide or mixed-metal oxide, or a conductive polymer. Examples ofsuitable metals include the Group 1 metals, the metals in Groups 4, 5,6, and the Group 8-10 transition metals. If the anode layer is to belight-transmitting, mixed-metal oxides of Group 12, 13, and 14 metals oroxides of combinations thereof, such as Au, Pt, and indium-tin-oxide(ITO), may be used. The anode layer may include an organic material suchas polyaniline, e.g., as described in “Flexible light-emitting diodesmade from soluble conducting polymer,” Nature, vol. 357, pp. 477-479(Jun. 11, 1992), graphene, e.g., as described in H. P. Boehm, R. Settonand E. Stumpp (1994). “Nomenclature and terminology of graphiteintercalation compounds”. Pure and Applied Chemistry 66: 1893-1901,and/or carbon nanotubes, e.g., Juni, et al, US App 20080152573(WO/2008/140505). Examples of suitable high work function electricalconductors include but are not limited to Au, Pt, indium-tin-oxide(ITO), or alloys thereof In an embodiment, the anode layer can have athickness in the range of about 1 nm to about 1000 nm.

Approximate work functions of some materials which may be useful in ananode are included in Table 2 below.

TABLE 2 Metal Work Function (eV) indium-tin-oxide (ITO) 4.7indium-zinc-oxide (IZO) 4.7 Al 4.28 Ag 4.26 Zn 4.33 Zr 4.05 Sn 4.42 V4.3 Hg 4.49 In 4.12 Ti 4.3

The series of organic layers includes a hole-transport layer, anelectron-transport layer, a first light-emitting layer, a secondlight-emitting layer, and may also include optional layers disposed inany position between these layers, or may be one or both of the outsidelayers which contact the anode or the cathode.

The hole-transport layer may be any layer which is capable oftransporting holes between the 2 layers it contacts. Examples of the 2layers that the hole-transport layer may contact include the anode andthe first light-emitting layer, a hole-injecting layer and the firstlight-emitting layer, the anode and an electron-blocking layer, theanode and an exciton-blocking layer, etc. The hole-transport layer mayinclude any material having HOMO and LUMO energy levels which aresuitable for a device described herein, and may include hole-transportmaterials known by those skilled in the art. In some embodiments, thehole-transport layer comprises a hole-transport compound having a HOMOenergy level and a LUMO energy level, wherein the HOMO energy level ofthe hole-transport layer is about equal to the HOMO energy level of thehole-transport compound, and the LUMO energy level of the hole-transportlayer is about equal to the LUMO energy level of the hole-transportcompound.

In some embodiments, the hole-transport compound may be anaromatic-substituted amine, a carbazole, a polyvinylcarbazole (PVK),e.g. poly(9-vinylcarbazole); polyfluorene; a polyfluorene copolymer;poly(9,9-di-n-octylfluorene-alt-benzothiadiazole); poly(paraphenylene);poly[2-(5-cyano-5-methylhexyloxy)-1,4-phenylene]; a benzidine; aphenylenediamine; a phthalocyanine metal complex; a polyacetylene; apolythiophene; a triphenylamine; an oxadiazole; copper phthalocyanine;1,1-Bis(4-bis(4-methylphenyl)aminophenyl)cyclohexane;2,9-Dimethyl-4,7-diphenyl-1,10-phenanthroline;3,5-Bis(4-tert-butyl-phenyl)-4-phenyl[1,2,4]triazole;3,4,5-Triphenyl-1,2,3-triazole;4,4′,4′-tris(3-methylphenylphenylamino)triphenylamine (MTDATA);N,N′-bis(3-methylphenyl)N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(TPD); 4,4′-bis[N-(naphthyl)-N-phenyl-amino]biphenyl (α-NPD);4,4′,4″-tris(carbazol-9-yl)-triphenylamine (TCTA);4,4′-bis[N,N′-(3-tolyl)amino]-3,3′-dimethylbiphenyl (HMTPD);4,4′-N,N′-dicarbazole-biphenyl (CBP); 1,3-N,N-dicarbazole-benzene (mCP);Bis [4-(p,p′-ditolyl-amino)phenyl]diphenylsilane (DTASi);2,2′-bis(4-carbazolylphenyl)-1,1′-biphenyl (4CzPBP);N,N′N″-1,3,5-tricarbazoloylbenzene (tCP);N,N′-bis(4-butylphenyl)-N,N-bis(phenyl)benzidine; or the like.

Approximate LUMO energy levels and HOMO energy levels of some materialswhich may be useful as hole-transport material are included in Table 3below.

TABLE 3 Hole-transport LUMO energy HOMO energy compound level (eV) level(eV) DTASi −2.20 −5.60 TCTA −2.43 −5.83 CBP −2.80 −6.10 α-NPD −2.40−5.50 4CzPBP −2.60 −6.06

In some embodiments, the hole-transport layer is formed of material thatalso functions as an electron blocking layer. In some embodiments, thehole-transport layer comprises material that also enables it to functionas an exciton blocking layer.

The electron-transport layer may be any layer which is capable oftransporting electrons between the 2 layers it contacts. Examples of the2 layers that the electron-transport layer may contact include thecathode and the second light-emitting layer, a electron-injecting layerand the second light-emitting layer, the cathode and a hole-blockinglayer, the cathode and an exciton-blocking layer, etc. Theelectron-transport layer may include any material having HOMO and LUMOenergy levels which are suitable for a device described herein, and mayinclude electron-transport materials known by those skilled in the art.In some embodiments, the electron-transport layer comprises anelectron-transport compound having a HOMO energy level and a LUMO energylevel, wherein the HOMO energy level of the electron-transport layer isabout equal to the HOMO energy level of the electron-transport compound,and the LUMO energy level of the electron-transport layer is about equalto the LUMO energy level of the electron-transport compound.

In some embodiments, the electron-transport compound may be2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD);1,3-bis(N,N-t-butyl-phenyl)-1,3,4-oxadiazole (OXD-7),1,3-bis[2-(2,2′-bipyridine-6-yl)-1,3,4-oxadiazo-5-yl]benzene;3-phenyl-4-(1′-naphthyl)-5-phenyl-1,2,4-triazole (TAZ);2,9-dimethyl-4,7-diphenyl-phenanthroline (bathocuproine or BCP);aluminum tris(8-hydroxyquinolate) (Alq3);1,3,5-tris(2-N-phenylbenzimidazolyl)benzene (TPBI); 1,3-bis[2-(2,2′-bipyridine-6-yl)-1,3,4-oxadiazo-5-yl]benzene (BPY-OXD), or thelike.

The LUMO energy level and HOMO energy level of some materials which maybe useful as electron-transport material are included in Table 3 below.

TABLE 3 Electron-transport LUMO energy HOMO energy material level (eV)level (eV) TPBI −2.70 −6.20 PBD −2.60 −6.20 OXD-7 −2.60 −6.40 TAZ −2.70−6.30 AlQ3 −3.00 −5.70 BCP −2.80 −6.10

In other embodiments, the energy difference between the LUMO of thematerial(s) that can be included in the electron-transport layer and thework function of the cathode layer is small enough to allow efficientelectron transport from the cathode. In some embodiments, theelectron-transport layer is formed of material that also functions as ahole blocking layer. In some embodiments, the electron-transport layercomprises material that also enables it to function as an excitonblocking layer.

The first host material of the first light-emitting layer may be atleast one of: one or more hole-transport materials, one or moreelectron-transport materials, and one or more ambipolar materials, whichinclude materials understood by those skilled in the art to be capableof transporting both holes and electrons. In some embodiments, the firsthost material comprises a phosphorescent material characterized by atriplet energy (e.g. the energy of a photon emitted as a result ofrelaxation of the material from the lowest triplet state to the groundstate) which is greater than the triplet energy of the first emissivematerial. In some embodiments, the first host material is at least about50%, at least about 90%, at least about 95%, or at least about 99% ofthe first light-emitting layer by weight, wherein the HOMO energy levelof the first light-emitting layer is about equal to a HOMO energy levelof the first host material, and the LUMO energy level of the firstlight-emitting layer is about equal to a LUMO energy level of the firsthost material.

The second host material of the second light-emitting layer may be atleast one of: one or more hole-transport materials, one or moreelectron-transport materials, and one or more ambipolar materials. Insome embodiments, the second host material comprises a phosphorescentmaterial characterized by a triplet energy which is greater than thetriplet energy of the second emissive material. In some embodiments, thesecond host material is at least about 50%, at least about 90%, at leastabout 95%, or at least about 99% of the second light-emitting layer byweight, wherein the HOMO energy level of the second light-emitting layeris about equal to a HOMO energy level of the second host material, andthe LUMO energy level of the second light-emitting layer is about equalto a LUMO energy level of the second host material.

The LUMO energy level and HOMO energy level of some materials that maybe useful as a first host material and/or a second host material areincluded in Table 4 below.

TABLE 4 Host LUMO energy HOMO energy material level (eV) level (eV)4CzPBP −2.60 −6.06 HO-1 −3.10 −6.08 HO-2 −2.88 −5.96

In some embodiments, the first host material and/or the second hostmaterial may comprise 4CzPBP, HO-2, or HO-1.

In some embodiments, the hole-transport layer comprises DTASi, the firsthost comprises 4CzPBP, the second host comprises HO-2, and theelectron-transport layer comprises TPBI. In some embodiments, thehole-transport layer comprises DTASi, the first host comprises HO-1, thesecond host comprises HO-2, and the electron-transport layer comprisesTPBI.

Any suitable emissive material may be used for the first emissivematerial or the second emissive material. In some embodiments, the firstemissive material emits visible photons that have a lower averagewavelength than visible photons emitted by the second emissive material.The term “average wavelength” has the ordinary meaning understood by aperson of ordinary skill in the art. In some embodiments, the averagewavelength includes the wavelength wherein, in an emission spectrum ofthe material, the area under the curve in the visible range atwavelengths lower than the average wavelength is about equal to the areaunder the curve in the visible range at wavelengths higher than theaverage wavelength.

In some embodiments, the first light-emitting layer emits visiblephotons having an average wavelength in the range of about 400 nm toabout 550 nm. In some embodiments, the second light-emitting layer emitsvisible photons having an average wavelength in the range of about 500nm to about 750 nm. In some embodiments, the first light-emitting layerand/or the second light-emitting layer comprises an iridium coordinationcompound such as bis(2-[4,6-difluorophenyl]pyridinato-N,C2′)iridium(III) picolinate (FIrPic),bis-{2-[3,5-bis(trifluoromethyl)phenyl]pyridinato-N,C2′}iridium(III)-picolinate,bis(2-[4,6-difluorophenyl]pyridinato-N,C2′)iridium(acetylacetonate),Iridium (III)bis(4,6-difluorophenylpyridinato)-3-(trifluoromethyl)-5-(pyridine-2-yl)-1,2,4-triazolate,Iridium (III)bis(4,6-difluorophenylpyridinato)-5-(pyridine-2-yl)-1H-tetrazolate, andbis[2-(4,6-difluorophenyl)pyridinato-N,C^(2′)]iridium(III)tetra(1-pyrazolyl)borate,Bis [2-(2′-benzothienyl)-pyridinato-N,C3′]iridium(III)(acetylacetonate); Bis[(2-phenylquinolyl)-N,C2′]iridium (III)(acetylacetonate); Bis[(1-phenylisoquinolinato-N,C2′)]iridium (III)(acetylacetonate); Bis[(dibenzo[f, h]quinoxalino-N,C2′)iridium(III)(acetylacetonate);Tris(2,5-bis-2′-(9′,9′-dihexylfluorene)pyridine)iridium (III);Tris[1-phenylisoquinolinato-N,C2′]iridium (III);Tris-[2-(2′-benzothienyl)-pyridinato-N,C3′]iridium (III);Tris[1-thiophen-2-ylisoquinolinato-N,C3′]iridium (III); andTris[1-(9,9-dimethyl-9H-fluoren-2-yl)isoquinolinato-(N,C3′)iridium(III)), Bis(2-phenylpyridinato-N,C2′)iridium(III)(acetylacetonate)[Ir(ppy)₂(acac)],Bis(2-(4-tolyl)pyridinato-N,C2′)iridium(III)(acetylacetonate)[Ir(mppy)₂(acac)], Bis(2-(4-tert-butyl)pyridinato-N,C2′)iridium(III)(acetylacetonate) [Ir(t-Buppy)₂(acac)],Tris(2-phenylpyridinato-N,C2′)iridium (III) [Ir(ppy)₃],Bis(2-phenyloxazolinato-N,C2′)iridium (III) (acetylacetonate)[Ir(op)₂(acac)], Tris(2-(4-tolyl)pyridinato-N,C2′)iridium(III)[Ir(mppy)₃]. etc.

In some embodiments, the second light-emitting layer comprises a yellowemitter and/or a red emitter. Examples of yellow emitters may include,but are not limited to, YE-1, Bis[(2-phenylquinolyl)-N,C2]iridium (III)(acetylacetonate), Ir(pq)₂acac;Bis[2-phenylbenzothiazolato-N,C2′]iridium (III)(acetylacetonate),(bt)₂Ir(III)(acac);Bis[2-(4-tert-butylphenyl)benzothiazolato-N,C2′]iridium(III)(acetylacetonate),(t-bt)₂Ir(III)(acac); Bis[(2-(2′-thienyl)pyridinato-N,C3′)]iridium (III)(acetylacetonate), (thp)₂Ir(III)(acac);Tris[2-(9.9-dimethylfluoren-2-yl)pyridinato-(N,C3′)]iridium (III),[Ir(Flpy)₃]; Tris[2-(9.9-dimethylfluoren-2-yl)pyridinato-(N,C3′)]iridium(III), [Ir(Flpy)₃]; Bis[5-trifluoromethyl-2-[3-(N-phenylcarbzolyl)pyridinato-N,C2′]iridium(III)(acetylacetonate),(Cz-CF₃)Ir(III)(acac); (2-PhPyCz)₂Ir(III)(acac).

Examples of red emitters may include, but are not limited to,Ir(piq)₂acac, Bis[2-(2 ′-benzothienyl)-pyridinato-N,C3′]iridium(III)(acetylacetonate); Bis[(2-phenylquinolyl)-N,C2′]iridium (III)(acetylacetonate); Bis[(1-phenylisoquinolinato-N,C2′)]iridium (III)(acetylacetonate); Bis[(dibenzo[f, h]quinoxalino-N,C2′)iridium(III)(acetylacetonate);Tris(2,5-bis-2′-(9′,9′-dihexylfluorene)pyridine)iridium (III);Tris[1-phenylisoquinolinato-N,C2′]iridium (III);Tris-[2-(2′-benzothienyl)-pyridinato-N,C3′]iridium (III);Tris[1-thiophen-2-ylisoquinolinato-N,C3′]iridium (III); andTris[1-(9,9-dimethyl-9H-fluoren-2-yl)isoquinolinato-(N,C3′)iridium(III)), etc.

1. (Btp)₂Ir(III)(acac); Bis[2-(2′-benzothienyl)-pyridinato-N,C3′]iridium(III)(acetylacetonate) 2. (Pq)₂Ir(III)(acac);Bis[(2-phenylquinolyl)-N,C2′]iridium (III)(acetylacetonate) 3.(Piq)₂Ir(III)(acac); Bis[(1-phenylisoquinolinato-N,C2′)]iridium(III)(acetylacetonate) 4. (DBQ)₂Ir(acac); Bis[(dibenzo[f,h]quinoxalino-N,C2′)iridium (III)(acetylacetonate) 5. [Ir(HFP)₃],Tris(2,5-bis-2′-(9′,9′-dihexylfluorene)pyridine)iridium (III) 6.Ir(piq)₃; Tris[1-phenylisoquinolinato-N,C2′]iridium (III) 7. Ir(btp)₃;Tris-[2-(2′-benzothienyl)-pyridinato-N,C3′]iridium (III) 8. Ir(tiq)₃,Tris[1-thiophen-2-ylisoquinolinato-N,C3′]iridium (III) 9. Ir(fliq)₃;Tris[1-(9,9-dimethyl-9H-fluoren-2-yl)isoquinolinato-(N,C3′)iridium(III))

The HOMO and LUMO energy levels for organic materials to be used inOLEDs may be obtained by several conventional methods known in the art,e.g. solution electrochemistry, ultraviolet photoelectron spectroscopy(UPS), inverse photoemission spectroscopy, etc. In some embodiments,HOMO and LUMO energy levels for organic materials to be used in OLEDsmay be obtained using a cyclic voltammetry (CV) instrument (modelμAutolab type II) manufactured by Metrohm USA (Riverview, Fla., USA) inconjunction with GPES/FRA software (version 4.9).

The devices described herein may provide an improved luminescentefficiency, a better color rendering index, and/or improved colorstability as compared to devices currently known in the art.

In some embodiments, the devices provided herein emit white light,meaning light which appears white to an ordinary observer. In someembodiments, white light is light having the approximate CIE colorcoordinates (X=1/3, Y=1/3). CIE color coordinates are known in the artto be useful to quantify the color of emitted light, and CIE colorcoordinates (X=1/3, Y=1/3) is known as the achromatic point. The X and Ycolor coordinates are weights applied to the CIE primaries to match acolor. A more detailed description of these terms may be found in CIE1971, International Commission on Illumination, Colorimetry: OfficialRecommendations of the International Commission on Illumination,Publication CIE No. 15 (E-1.3.1) 1971, Bureau Central de la CIE, Paris,1971 and in F. W. Billmeyer, Jr., M. Saltzman, Principles of ColorTechnology, 2nd edition, John Wiley & Sons, Inc., New York, 1981, bothof which are hereby incorporated by reference in their entireties. Thecolor rendering index (CRI) refers to the ability to render variouscolors and has values ranging from 0 to 100, with 100 being the best.

EXAMPLE 1

The devices were generally fabricated as follows.

ITO coated glass substrates were cleaned by ultrasound in acetone, andconsecutively in 2-propanol, baked at about 110° C. for about 3 hours,followed by treatment with oxygen plasma for about 30 min. A layer ofPEDOT: PSS (Baytron P from H. C. Starck) was spin-coated at about 6000rpm onto the pre-cleaned and O₂-plasma treated (ITO)-substrate andannealed at about 180° C. for about 30 min, yielding a thickness ofaround 20 nm. In a glove-box hosted vacuum deposition system at apressure of about 10⁻⁷ torr (1 torr=133.322 Pa),4,4′4″-tri(N-carbazolyl)triphenylamine (TCTA) was first deposited on topof PEDOT/PSS layer at deposition rate of about 0.06 nm/s, yielding a 40nm thick film. Then the first emitter (about 12% wt) with the first hostwas co-deposited on top of TCTA to form a 5-10 nm thick film as firstlight-emitting layer, followed by co-deposition of the second emitter(about 5% wt) and the second host to form a 1-5 nm thick film as secondlight-emitting layer. A third light-emitting layer (if necessary) may beprepared by co-deposition of the third emitter and the third host toform a 5-10 nm thick film. A 40 nm thick layer of1,3,5-tris(N-phenylbenzimidizol-2-yl)benzene (TPBI) was then depositedon top of the second light-emitting layer at deposition rate around 0.06nm/s. LiF and Al were then deposited successively at deposition rates of0.005 and 0.2 nm/s, respectively.

EXAMPLE 2

The following device, which has the conventional stepwise decrease inHOMO energy levels and LUMO energy levels, was prepared according to theprocedure of Example 1.

LUMO Emissive Layer Material (eV) HOMO (eV) Material Hole-transportDTASi −2.20 −5.60 NA First light-emitting 4Cz-PBP −2.60 −6.06 FIrPicSecond light-emitting 4Cz-PBP −2.60 −6.06 Ir(pq)2acac Thirdlight-emitting 4Cz-PBP −2.60 −6.06 FIrPic Electron-transport TPBI −2.70−6.20 NA

The performance of this device was evaluated by I-V measurements using aKeithley 2400 Source Meter to apply 0-10 V voltage scans and measure thecurrent simultaneously. All device operations were done inside anitrogen-filled glove-box. FIG. 9 is a plot of the luminance efficiencyand the power efficiency of the device at varying brightness. FIG. 9shows that the luminance efficiency generally ranges from about 28 cd/Ato about 31 cd/A, and the power efficiency ranges from about 19 lm/W toabout 24 lm/W, at a brightness ranging from about 200 cd/m² to about1,000 cd/m² brightness. At 1,000 cd/m² brightness, driving under 4.5V(current density 3.4 mA/cm²), the device exhibits luminance efficiencyof 28.4 cd/A and power efficiency of 19.6 lm/W.

EXAMPLE 3

The following device, which has the conventional stepwise decrease inHOMO energy levels, was prepared according to the procedure of Example1.

LUMO Emissive Layer Material (eV) HOMO (eV) Material Hole-transportDTASi −2.20 −5.60 NA First light-emitting HO-1 −3.10 −6.08 FIrPic Secondlight-emitting HO-1 −3.10 −6.08 Ir(pq)2acac Third light-emitting HO-1−3.10 −6.08 FIrPic Electron-transport TPBI −2.70 −6.20 NA

The performance of this device was evaluated by I-V measurements using aKeithley 2400 Source Meter to apply 0-10 V voltage scans and measure thecurrent simultaneously. All device operations were done inside anitrogen-filled glove-box. FIG. 10 is a plot of the luminance efficiencyand the power efficiency of the device at varying brightness. FIG. 10shows that the luminance efficiency generally ranges from about 37 cd/Ato about 42 cd/A, and the power efficiency ranges from about 24 lm/W toabout 33 lm/W, at a brightness ranging from about 200 cd/m² to about1,000 cd/m² brightness. At 1,000 cd/m² brightness, driving under 3.6V,the device exhibits luminance efficiency of 37.0 cd/A and powerefficiency of 24.0 lm/W, which is more efficient than the device ofExample 2.

EXAMPLE 4

The following device was prepared according to the procedure of Example1.

LUMO HOMO Layer Material (eV) (eV) Emissive Material Hole-transportDTASi −2.20 −5.60 NA First light-emitting 4CzPBP −2.60 −6.06 FIrPicSecond light-emitting HO-2 −2.88 −5.96 Ir(piq)2acac, YE-1Electron-transport TPBI −2.70 −6.20 NA

The performance of this device was evaluated by I-V measurements using aKeithley 2400 Source Meter to apply 0-10 V voltage scans and measure thecurrent simultaneously. All device operations were done inside anitrogen-filled glove-box. FIG. 11 a is a plot of the luminanceefficiency and the power efficiency of the device at varying brightness.FIG. 11 a shows that the luminance efficiency generally ranges fromabout 58 cd/A to about 62 cd/A, and the power efficiency ranges fromabout 44 lm/W to about 50 lm/W, at a brightness ranging from about 200cd/m² to about 1,000 cd/m² brightness. At 1,000 cd/m² brightness,driving under 4.2V and current density of 1.7 mA/cm², the deviceexhibits luminance efficiency of 59.0 cd/A and power efficiency of 44.0lm/W, and is thus more efficient than the devices of Example 2 andExample 3. FIG. 11 b is a plot of the electroluminescence spectrum ofthe same device, showing strong emission in the visible range, with CIE(0.334, 0.396), CRI(74).

EXAMPLE 5

The following device was prepared according to the procedure of Example1.

LUMO HOMO Layer Material (eV) (eV) Emissive Material Hole-transportDTASi −2.20 −5.60 NA First light-emitting HO-1 −3.10 −6.08 FIrPic Secondlight-emitting HO-2 −2.88 −5.96 Ir(piq)2acac, YE-1 Electron-transportTPBI −2.70 −6.20 NA

The performance of this device was evaluated by I-V measurements using aKeithley 2400 Source Meter to apply 0-10 V voltage scans and measure thecurrent simultaneously. All device operations were done inside anitrogen-filled glove-box. FIG. 12 a is a plot of the luminanceefficiency and the power efficiency of the device at varying brightness.FIG. 12 a shows that the luminance efficiency generally ranges fromabout 66 cd/A to about 72 cd/A, and the power efficiency ranges fromabout 51 lm/W to about 70 lm/W, at a brightness ranging from about 200cd/m² to about 1,000 cd/m² brightness. At 1,000 cd/m² brightness,driving under 4.1V, the device exhibits luminance efficiency of 66.0cd/A and power efficiency of 51.0 lm/W, and is thus more efficient thanthe devices of Example 2 and Example 3. FIG. 12 b is a plot of theelectroluminescence spectrum of the same device showing strong emissionin the visible range, with CIE (0.35, 0.43), CRI(68).

Although this invention has been disclosed in the context of certainpreferred embodiments and examples, it will be understood by thoseskilled in the art that the present invention extends beyond thespecifically disclosed embodiments to other alternative embodimentsand/or uses of the invention and obvious modifications and equivalentsthereof. Thus, it is intended that the scope of the present inventionherein disclosed should not be limited by the particular disclosedembodiments described above, but should be determined only by a fairreading of the claims that follow.

1. An organic light-emitting device comprising: a cathode, an anode, anda series of organic layers disposed between the anode and the cathode,wherein the series of organic layers comprises: a first light-emittinglayer deposited between the anode and the cathode, wherein the firstlight-emitting layer comprises a first host material and a firstemissive material; a hole-transport layer, disposed between the anodeand the first light-emitting layer; a second light-emitting layerdisposed between the first light-emitting layer and the cathode, whereinthe second light-emitting layer comprises a second host material and asecond emissive material; and an electron-transport layer disposedbetween the second light-emitting layer and the cathode; wherein a HOMOenergy level of the hole-transport layer is higher than a HOMO energylevel of the electron-transport layer, and a LUMO energy level of thehole-transport layer is higher than a LUMO energy level of the electrontransport layer; and at least one of the following relationships exists:a HOMO energy level of the first light-emitting layer is higher than theHOMO energy level of the hole-transport layer; a LUMO energy level ofthe first light-emitting layer is lower than a LUMO energy level of thesecond light-emitting layer; a HOMO energy level of the secondlight-emitting layer is higher than the HOMO energy level of the firstlight-emitting layer; or the LUMO energy level of the secondlight-emitting layer is lower than a LUMO energy level of theelectron-transport layer.
 2. The organic light-emitting device of claim1, wherein the HOMO energy level of the second light-emitting layer ishigher than the HOMO energy level of the first light-emitting layer, andthe LUMO energy level of the first light-emitting layer is lower thanthe LUMO energy level of the second light-emitting layer.
 3. The organiclight-emitting device of claim 1, wherein the HOMO energy level of thesecond light-emitting layer is higher than the HOMO energy level of thefirst light-emitting layer, and the LUMO energy level of the secondlight-emitting layer is lower than the LUMO energy level of theelectron-transport layer.
 4. The organic light-emitting device of claim1, wherein the HOMO energy level of the second light-emitting layer ishigher than the HOMO energy level of the first light-emitting layer, theLUMO energy level of the first light-emitting layer is lower than theLUMO energy level of the second light-emitting layer, and the LUMOenergy level of the second light-emitting layer is lower than the LUMOenergy level of the electron-transport layer.
 5. The organiclight-emitting device of claim 1, wherein the hole-transport layercomprises a hole-transport compound having a HOMO energy level and aLUMO energy level, wherein the HOMO energy level of the hole transportlayer is about equal to the HOMO energy level of the hole-transportcompound, and the LUMO energy level of the hole-transport layer is aboutequal to the LUMO energy level of the hole-transport compound.
 6. Theorganic light-emitting device of claim 1, wherein the electron-transportlayer comprises an electron-transport compound having a HOMO energylevel and a LUMO energy level, wherein the HOMO energy level of theelectron-transport layer is about equal to the HOMO energy level of theelectron-transport compound, and the LUMO energy level of theelectron-transport layer is about equal to the LUMO energy level of theelectron-transport compound.
 7. The organic light-emitting device ofclaim 1, wherein the first host material is at least about 50% of thefirst light-emitting layer by weight, wherein the HOMO energy level ofthe first light-emitting layer is about equal to a HOMO energy level ofthe first host material, and the LUMO energy level of the firstlight-emitting layer is about equal to a LUMO energy level of the firsthost material.
 8. The organic light-emitting device of claim 1, whereinthe second host material is at least about 50% of the secondlight-emitting layer by weight, wherein the HOMO energy level of thesecond light-emitting layer is about equal to a HOMO energy level of thesecond host material, and the LUMO energy level of the secondlight-emitting layer is about equal to a LUMO energy level of the secondhost material.
 9. The organic light-emitting device of claim 1, whereinthe first emissive material emits visible photons that have a loweraverage wavelength than visible photons emitted by the second emissivematerial.
 10. The organic light-emitting device of claim 1, wherein thefirst light-emitting layer emits visible photons having an averagewavelength in the range of about 400 nm to about 550 nm.
 11. The organiclight-emitting device of claim 1, wherein the first light-emitting layercomprises an iridium coordination compound.
 12. The organiclight-emitting device of claim 11, wherein the iridium coordinationcompound is FIrPic.
 13. The organic light-emitting device of claim 1,wherein the second light-emitting layer comprises an iridiumcoordination compound.
 14. The organic light-emitting device of claim 1,wherein the second light-emitting layer emits visible photons having anaverage wavelength in the range of about 500 nm to about 750 nm.
 15. Theorganic light-emitting device of claim 1, wherein the secondlight-emitting layer comprises a yellow emitter and a red emitter. 16.The organic light-emitting device of claim 15, wherein the yellowemitter is YE-1 or Ir(pq)₂acac.
 17. The organic light-emitting device ofclaim 15, wherein the red emitter is Ir(piq)₂acac.
 18. The organiclight-emitting device of claim 1, wherein the hole-transport layercomprises DTASi, the first host comprises 4CzPBP, the second hostcomprises HO-2, and the electron-transport layer comprises TPBI.
 19. Theorganic light-emitting device of claim 1, wherein the hole-transportlayer comprises DTASi, the first host comprises HO-1, the second hostcomprises HO-2, and the electron-transport layer comprises TPBI.